As a method for producing a carbon fiber bundle, known is a method of obtaining a carbon fiber bundle by performing flameproofing treatment of subjecting a carbon fiber precursor acrylic fiber bundle to heat treatment in oxidizing atmosphere of 200 to 300° C., and then performing carbonization treatment of subjecting the obtained flameproof fiber bundle to heat treatment in inactive atmosphere of 1000° C. or higher. The carbon fiber bundle obtained in this method has excellent mechanical properties, and thus is industrially broadly used particularly as a reinforcing fiber for a composite material.
In the flameproofing process of performing flameproofing treatment for a carbon fiber precursor acrylic fiber bundle in preparing a carbon fiber bundle, fusion between filaments may cause process difficulties such as fluff and bundle breakage in the flameproofing process and the carbonization process that is subsequent to the flameproofing process (hereinafter, the flameproofing process and the carbonization process may be collectively called as the “baking process”.). In order to avoid this fusion, selection of oil being applied to the carbon fiber precursor acrylic fiber bundle is known to be important. Among the oils, a silicone-based oil containing silicone that is good in an effect of preventing fusion in the flameproofing process is most generally used (Patent Document 1).
In a flameproofing furnace in which flameproofing treatment is performed for a carbon fiber precursor acrylic fiber bundle, heated oxidizing gas is circulated with a fan. In this furnace, a portion of a silicone compound in silicone-based oil given to the carbon fiber precursor acrylic fiber bundle volatilizes into the oxidizing gas, and stays for a long time in the circulating gas. On the other hand, the residual portion of the silicone compound on the surface of the carbon fiber precursor acrylic fiber bundle achieves effects of preventing fusion of the filaments to each other, maintaining convergence of the carbon fiber precursor acrylic fiber bundle, and suppressing filament breakage. The silicone-based compound that volatilizes into oxidizing gas and stays for a long time in the flameproofing furnace may shortly solidify, and deposit in the furnace, and also adhere as particles to a fiber bundle in the flameproofing treatment. It is known that these particles adhering to the fiber bundle become a starting point for occurrence of fluff or occurrence of single yarn breakage in subsequent carbonization process, and remarkably lower the performances of the obtained carbon fiber. In addition, it is revealed that oil ingredients other than the silicone compound, tar ingredients derived from the carbon fiber precursor acrylic fiber bundle, dust from the outside of the furnace brought by the fiber bundle, dust contained in the air from the intake, and the like also adhere to the fiber bundle and are factors for lowering the strength of carbon fiber.
To resolve the problems described above, a technology is suggested in Patent Document 2 from a viewpoint of removing dust present in a flameproofing furnace, in which an exhaust port is arranged in an gas circulation path installed in a flameproofing furnace, and a portion of sucked gas is exhausted from the exhaust port with a circulation fan before starting operation of the flameproofing furnace, whereby to reduce and remove dust in the furnace.
On the other hand, a technology is suggested in Patent Documents 3 and 4 from a viewpoint of removing pitch and a tar-like substance and the like adhering to the surface of a fiber bundle in a process of preparing a carbon fiber bundle, wherein a flameproof fiber bundle is subjected to ultrasonic treatment in a liquid containing a surfactant, whereby to remove pitch and a tar-like substance and the like adhering to the surface of the fiber bundle, and to allow subsequent uniform carbonization, and thus to obtain a carbon fiber bundle excellent in the strength with short time flameproofing treatment.
However, the technology disclosed in Patent Document 2 needs to be performed in the state where the operation of preparing a carbon fiber bundle is stopped, and stability of a long-term continuous operation of a flameproofing furnace cannot be expected. In addition, with the technologies disclosed in Patent Document 3, it is difficult to effectively remove particles of silicon oxide derived from the silicone-based oil and the like in the inside of the fiber bundle that is an assembly of thousands to tens of thousands of filaments. In addition, the technologies disclosed in Patent Documents 3 and 4 use wet washing treatment in order to remove deposits on the surface of a fiber bundle, and inevitably need a drying treatment process of the fiber bundle, and are undesirable economically.